1. Field of the Invention
This invention generally relates to a motor driving circuit. This invention specifically relates to a driving circuit for a brushless dc motor used in electrical equipment, such as a tape recorder and a video tape recorder (VTR).
2. Description of the Prior Art
Ways of driving a dc motor are conventionally of two types, that is, the constant-voltage type and the constant-current type. Structures for driving a dc motor are conventionally of two types, that is, the series-regulator type and the switching-regulator type. Since the switching-regulator type is more efficient than the series-regulator type, the former is widely used.
FIG. 1 shows a conventional motor driving circuit of the constant-voltage switching-regulator type which uses pulse-width modulation (PWM).
In the circuit of FIG. 1, a brush-less dc motor 1 includes three-phase coils or windings 2, 3, and 4, and a rotor (not shown). A generator 5 is associated with the rotor to generate a signal indicative of the rotational speed of the rotor. This rotational speed signal is fed to a servo circuit 8 via an amplifier 7. The servo circuit 8 generates a control signal Sc which depends on the rotational speed of the rotor. This control signal Sc is applied to a PWM circuit 10 via a linear amplifier 9. The PWM circuit 10 generates a pulse signal whose pulse width varies as a function of the rotational speed of the rotor. The pulse signal having PWM pulses is applied to the base of a power driving transistor 11 so that the transistor 11 is switched on and off in response to the PWM pulses. Thus, the voltage at the collector of the transistor 11 is in the form of pulses reflecting the PWM pulses. The emitter of the transistor 11 is connected to the positive terminal of a constant voltage source (not shown). The collector of the transistor 11 is connected through a coil 13 to a common junction O of the motor windings 2, 3, and 4. The motor windings 2, 3, and 4 are connected to the negative terminal of the constant voltage source, that is, to the ground, via switches 17, 18, and 19 respectively in a switch circuit 16. The cathode of a flywheel diode 12 is connected to the junction between the transistor 11 and the coil 13. The anode of the flywheel diode 12 is grounded. One terminal of a capacitor 14 is connected to the common junction O of the motor windings 2, 3, and 4. The other terminal of the capacitor 14 is grounded. The combination of the coil 13 and the capacitor 14 constitutes a smoothing circuit which converts the collector pulse voltage to substantially a constant voltage applied to the common junction O of the motor windings 2, 3, and 4. It should be noted that the level of the voltage applied to the common junction O depends on the rotational speed of the rotor, since the width of the pulses from the PWM circuit 10 varies as a function of the rotational speed of the rotor. In operation, the coil 13 develops counterelectromotive force. The flywheel diode 12 allows the counterelectromotive force to be used. Specifically, the counterelectromotive current flows through the diode 12 and the capacitor 14 so that the capacitor 14 is charged with the current.
An angular position sensor 6 is associated with the rotor of the motor 1 to generate a signal indicative of the angular position of the rotor. This position signal is applied to a control circuit 15. The control circuit 15 generates control pulse signals in sequence depending on the angular position of the rotor. These control pulse signals are applied to the switch circuit 16 so that the switches 17, 18, and 19 are closed sequentially for a predetermined interval in terms of the angle of the rotor. Accordingly, currents flow through the motor windings 2, 3, and 4 in turn.
In the case of VTR's and tape recorders, variations in the amount of recorded or non-recorded portions of tape cause changes in the load on the motor 1 which would result in variations in the torque output of the motor 1. A servo or feed-back loop including the servo circuit 8 adequately removes and prevents the variations in the motor torque output caused by the tape amount variations. Switching action of the switches 17, 18 and 19 causes ripples to develop in the motor torque output for the following reason. At moments of switching of the switches 17, 18, and 19, an inadequate level of current flows through the motor windings 2, 3, and 4, reducing the motor torque output. This reduction of the motor torque output causes a torque ripple. In the case where the motor 1 constitutes a drum motor of a VTR, such a motor torque ripple results in adverse phenomena, for example, jitter.